Probability of precipitation means what?

Answer 1

Forecasts issued by the National Weather Service routinely include a "PoP" (probability of precipitation) statement, which is often expressed as the "chance of rain" or "chance of precipitation".

EXAMPLEZONE FORECASTS FOR NORTH AND CENTRAL GEORGIA

NATIONAL WEATHER SERVICE PEACHTREE CITY GA

119 PM EDT THU MAY 8 2008

GAZ021-022-032034-044046-055-057-090815-

CHEROKEE-CLAYTON-COBB-DEKALB-FORSYTH-GWINNETT-HENRY-NORTH FULTON-

ROCKDALE-SOUTH FULTON-

INCLUDING THE CITIES OF...ATLANTA...CONYERS...DECATUR...

EAST POINT...LAWRENCEVILLE...MARIETTA

119 PM EDT THU MAY x 2008

.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF

SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR

STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH.

.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND

THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS

AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST

WINDS 5 TO 15 MPH. CHANCE OF RAIN 40 PERCENT.

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point you select in the area.

How do forecasters arrive at this value?

Mathematically, PoP is defined as follows: PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, there is a correct way to interpret the forecast: According to the NWS, "CHANCE OF RAIN 40 PERCENT" means there is a 40 percent chance that rain will occur at any given point in the area.

NOTE! However, this is not entirely accurate language. It is better expressed as: "CHANCE OF RAIN 40 PERCENT" means the on average for all of the points in the area during the specified time period (usually 12 hour periods), chance that rain will occur (on average) is 40%.

Explanation:Suppose the the forecast were for Maui, HI. One "given" point is your house near the top of Mt. Haleakala, where it rains almost constantly. 40% percent is obviously not accurate for that given point. So assume that Mt. Haleakala is 10% of the area of Maui and that the average chance of rain today there is 80%. And assume that the average chance of rain for the other 90% of the island is 35%. So for the entire island, the average chance of rain is (.9 * .35) + (.1 * .8) = .4 = 40%.

Clearly, the smaller the area, the more meaningful and accurate "chance of rain" is.